1. Field of the Invention
A typical power cutter comprises a housing in which is mounted a two stroke internal combustion engine. Attached to the side of the housing is a support arm which extends forward of the housing. Rotatably mounted on the end of the support arm is a cutting blade, usually in the form of a grinding disk. The motor is drivingly connected to the cutting blade via a drive belt. The rotary output of the engine rotatingly drives the cutting blade via the drive belt. The drive belt is driven via a centrifugal clutch which enables the out drive spindle of the engine to disengage from the belt when the engine is running at a slow speed, to allow the engine to continue running, whilst disengaging any drive to the cutting blade to allow the blade to be stationary.
Also mounted in the housing is a fuel tank which provides fuel for the engine via a carburetor. An oil tank can also be provided, which provides lubricating oil to mix with the fuel, to lubricate the engine.
Mounted on the rear of the housing is a rear handle for supporting the power cutter, which contains a trigger switch for accelerating the engine upon depressing. Depression of the trigger switch causes more of the aerated fuel/oil mixture to be injected into the engine which in turn causes the speed of the engine to accelerate.
GB2232913 and WO2005/056225 show examples of such power cutters.
2. Description of the Related Art
A known problem with all two stokes engines is the lubrication of the crank shaft 114 (using the reference numbers shown in FIG. 19) and piston 1000. This is due to the fact that the fuel/air mixture first passes through a chamber 18 in the cylinder 120 below the piston 1000 before being forced into the chamber 122 above the piston 16 before being ignited. This prevents the use of lubricating oil being pumped around the crank shaft 114 on the underside of the piston 1000. Therefore, in existing designs of two stoke engines, a mixture of aerated fuel and lubricating oil is burnt within the engine, the oil providing lubrication for the crank shaft 114 and piston 1000 prior to being burnt with the fuel during the combustion cycle. A common problem with the use of two stoke engines is that the operator forgets to add the oil to the fuel or adds it in an inconsistent manner, thus resulting in damage to the engine. This particularly so for power cutters as often power cutters are often hired out to operators. As such, operators are not familiar with using them; they are not in the operator's possession for long; the operator does not know who had it previously or how the previous operator used the power cutter; and, as the operator does not own it, has less reason to care for the long term maintenance of the power cutter. Therefore, it is desirable that a mechanism is provided to prevent or limit damage to the two stroke engine of the power cutter to ensure that sufficient lubrication oil is included in the aerated fuel which prior to entering the engine.
It is possible to add lubricating oil directly to the fuel in the fuel tank. However, this relies on the operator ensuring that the correct ratio of lubricating oil to fuel is achieved. This can be difficult as it may be hard to determine how much fuel is already in the fuel tank and how much lubricating oil has already been added. Therefore, it is preferable to have a separate oil tank which is filled with lubricating oil and which is then pumped from the tank and mixed with the fuel. This enables the amount of oil be added to the fuel to be controlled more accurately.
The two stroke engines of power cutters use a carburetor to provide aerated fuel for powering the engine. A typical design of such a carburetor is shown in FIG. 31. Referring to FIG. 31, the carburetor includes a housing 1002 through which is formed an air passageway 1004 through which air can pass in the direction of Arrow Q. The air passageway 1004 at its entrance and exit has the same cross-sectional area. However, formed part way along the length of the air passageway 1004, is a restriction 1006 which reduces the size of the cross sectional area of the air passageway and which acts as a venturi; the air passageway 1004 narrowing and then expanding as the air passes through it. This causes the rate of flow of air through the narrow section 1008 of the air passageway 1004 to increase.
Fuel enters the carburetor via an inlet 1010 and fills a first chamber 1012. The first chamber 1012 is connected to a second chamber 1014 via a fuel passageway 1016. Fuel fills the second chamber 1014 via the fuel passageway 1016.
An adjustable needle valve 1020, which has a pointed tip and an elongate body, is mounted in the fuel passageway 1016 and which can axially slide within the passage way 1016. The fuel passageway 1016 includes a narrow section 1018. The tip of the adjustable needle valve 1020 projects towards the narrow section 1018 and can block the narrow section 1018 when the adjustable needle valve 1020 is moved towards it or open the narrow section 1018 when it is moved away from it.
The rear end, remote from the tip, of the adjustable needle valve 1020 projects into the second chamber 1014. Attached to the rear end of the adjustable needle valve 1020 is a first lever 1022 which connects to a second lever 1024 via a pivot point 1026. One of the walls of the second chamber 1014 is a flexible diaphragm 1032 which can move to adjust the volume of the second chamber 1014. A hollow chamber 1034 is formed on the other side of the diaphragm 1032. The end of the second lever 1024 connects to the diaphragm 1032. The second lever 1024 also connects to a solid wall 1028 of the second chamber 1014 via a spring 1030. The spring 1030 biases the second lever to a predetermine position, which in turn biases the first lever 1022 to a predetermined angular position. Flexing of the diaphragm 1032 causes pivotal movement of the first and second levers against the biasing force of the spring 1030. Movement of the first lever 1022 causes an axial sliding movement of the adjustable needle valve 1020 moving its tip towards or away from the narrow section 1018.
A first passageway 1036 connects to the second chamber 1014 via a high speed needle valve 1038. The other end of the first passageway 1036 connects with the narrow section 1008 of the air passageway 1004. A second passageway 1040 connects to the second chamber 1014 via an idle needle valve 1042. The other end of the second passageway 1040 connects via three small vents 1044 with the air passageway 1004 down stream of the narrow section 1008. The high speed needle valve 1038 is preset and limits the rate of flow of fuel through the first passageway 1036. The idle needle valve 1042 is preset and limits the rate of flow of fuel through the second passageway 1040.
Located in the air passageway 1004, ahead of the narrow section 1008, is a first pivotal plate 1046 which acts as the choke for the carburetor. The plate 1046 can be pivoted between an open position (as shown) where it extends in the direction of the air passageway 1004, allowing the maximum amount of air to enter the passage 1004, to a closed position where it extends across the air passageway 1004, substantially reducing the amount of air able enter the air passageway 1004.
Located in the air passageway 1004, downstream of the narrow section 1008, is a second pivotal plate 1048 which acts as the throttle for the carburetor. The plate 1048 can be pivoted between an open position where it extends in the direction of the air passage 1004, allowing the maximum amount of air to leave the air passageway 1004, to a closed position where it extends across the air passageway 1004, substantially reducing the amount of air able to leave the air passageway 1004. The plate 1048 is shown half way between its open and closed positions.
When the carburetor is in normal use, the first pivotal plate 1046 is in its open position. Air is drawn through the air passageway passing through the narrow section 1008 which causes it to speed up. The movement of the air through the narrow section 1008 causes fuel to be drawn out of the first passageway 1036 into the air flow and then pass through the air passageway 1004. The amount of air, and hence the amount of fuel drawn out of the first passageway 1036, is dependent on the angular position of the second pivotal plate 1048. When it is in its open position, the maximum amount of air is able to pass through the air passageway, drawing out the maximum amount of fuel from the first passageway 1036. When it is in its closed position, the minimum amount of air is able to pass through the air passageway, drawing out the minimum amount of fuel from the first passageway 1036. In order to ensure that sufficient fuel enters the air flow in the air passageway 1004 when the second pivotal plate 1048 is in its closed position, the second passageway 1040 also provides fuel to the air flow. However, the exit of the second passageway 1040 connects to the air passageway down stream of the second pivotal plate 1048 to ensure that there is always sufficient fuel entering the air flow.
As fuel is drawn out of the two passageways 1036; 1040, the amount of fuel in the second chamber 1014 reduces. When the amount of fuel reduces, the diaphragm 1032 flexes, to reduce the volume of the second chamber 1014 to accommodate the loss of fuel. As the diaphragm 1032 flexes, it moves the first and second pivotal levers 1024; 1022 against the biasing force of the spring 1030, which in turn axially slides the adjustable needle valve 1020, moving its tip away from the narrow section 1018, opening it up and allowing fuel to flow from the first chamber 1012 into the second chamber 1014. As the second chamber 1014 fills up, the diaphragm 1032 flexes to accommodate the additional fuel, pivoting the levers and moving the tip of the adjustable needle valve 1020 towards the narrow section 1018 and reducing the amount of fuel flowing through the fuel passageway. Movement of the diaphragm 1032 ensures movement of the tip of the adjustable needle valve 1020 relative to the narrow section 1018 is controlled to limit the amount of fuel in the second chamber 1014.
When the engine is cold, the first pivotal plate 1046 is placed in its closed position. This reduces the amount of air entering the air passageway 1004 and therefore provides a higher ratio of fuel to air in the air passageway to enable the cold engine to run.
The angular position of the first pivotal plate 1046 is set using a Bowden cable connected to a separate lever which is adjusted manually by the operator of the power cutter. The angular position of the second pivotal plate 1048 is set using a Bowden cable connected to a trigger switch 1070 mounted on the handle which is manually adjusted by the operator.
A problem with this design of carburetor on a power cutter is that the operator has to be constantly adjusting the angular position of the first pivotal plate when the engine is cold to ensure the smooth operation of the engine. This is particularly difficult if the operator is also trying to use the power cutter.